A conventional method of actuating a valve, such as, for example, a fuel injector is by use of an electromechanical solenoid arrangement. The solenoid is typically an insulated conducting wire wound to form a tight helical coil. When current passes through the wire, a magnetic field is generated within the coil in a direction parallel to the axis of the coil. The resulting magnetic field exerts a force on a moveable ferromagnetic armature located within the coil, thereby causing the armature to move a needle valve into an open position in opposition to a force generated by a return spring. The force exerted on the armature is proportional to the strength of the magnetic field; the strength of the magnetic field depends on the number of turns of the coil and the amount of current passing through the coil.
In the conventional fuel injector, the point at which the armature, and therefore the needle, begins to move varies primarily with the spring preload holding the injector closed, the friction and inertia of the needle, fuel pressure, eddy currents in the magnetic materials, and the magnetic characteristics of the design, e.g., the ability to direct flux into the working gap. Generally, the armature will not move until the magnetic force builds to a level high enough to overcome the opposing forces. Likewise, the needle will not return to a closed position until the magnetic force decays to a low enough level for the spring to overcome the fuel flow pressure and needle inertia. In a conventional injector design, once the needle begins opening or closing, it may continue to accelerate until it impacts with its respective end-stop, creating wear in the needle valve seat, needle bounce, and unwanted vibrations and noise problems.
Another conventional method of actuating a valve such as, for example, a fuel injector is by use of a piezoelectric actuator comprising a stack of piezoceramic or piezocrystal wafers bonded together to form a piezostack transducer. The piezostack transducer is operatively attached to the needle valve or similar member. Transducers convert energy from one form to another and the act of conversion is referred to as transduction. The piezoelectric transducer converts energy in an electric field into a mechanical strain in the piezoelectric material. Accordingly, when the piezostack has a high voltage potential applied across the wafers, the piezoelectric effect causes the stack to change dimension. This dimensional change in the piezostack may be used to actuate the needle valve.
The piezostack applies full force during the armature travel, allowing for controlled trajectory operation, and the characteristic ultrasonic operation of the piezostack provides good fuel atomization. However, the piezostack may fail to function when exposed to fuel or other engine fluids. Thus, in order to enable the piezostack to function properly, additional injector components may be required to isolate the piezostack from the engine environment and fuel, while allowing the useful motion of the piezostack to remain operatively coupled to the injector valve.
Yet another method of actuating a valve, such as a fuel injector is by use of a magnetostrictive member that changes length in the presence of a magnetic field. The dimensional changes that occur when a ferromagnetic material is placed in a magnetic field are normally considered undesirable effects because of the need for dimensional stability in precision electromagnetic devices. Therefore, manufacturers of ferromagnetic alloys often formulate their alloys to exhibit very low magnetostriction. However, ferromagnetic materials exhibit magnetic characteristics because of their ability to align magnetic domain. Strongly magnetostrictive materials characteristically have magnetic anisotropy closely coupled with magnetostrictive anisotropy, thus allowing the domains to change the major dimensions of the ferromagnetic material when the domains rotate. The magnetostriction materials are, in practice, not sensitive to field polarity, thereby giving the same magnitude of extension regardless of the polarity of the magnetic field, which is dissimilar to a piezostack transducer in that the piezostack is sensitive to the polarity of the electric field being applied to the piezostack.
The alloying of the elements Terbium (Tb), Dysprosium (Dy), and Iron (Fe) to form TbxDy1-xFey allowed for useful strains to be attained. For example, the magnetostrictive alloy Terfenol-D (Th0.32Dy0.68Fe1.92) is capable of approximately 10 um displacements for every 1 cm of length exposed to an approximately 500 Oersted magnetizing field. The general equation for magnetizing force, H, in Ampere-Tums per meter (1 Oersted=79.6 AT/m) is:                H=IN/L, where I=Amperes of current; N=number of turns; and L=path length.        
Terfenol-D is often referred to as a “smart material” because of its ability to respond to its environment and exhibit giant magnetostrictive properties. The present invention will be described primarily with reference to Terfenol-D as a preferred magnetostrictive material. However, it will be appreciated by those skilled in the art that other alloys having similar magnetostrictive properties may be substituted and are included within the scope of the present invention.
In the aforementioned methods of actuating a fuel injector, various materials are typically used, each having a unique coefficient of thermal expansion. Accordingly, thermal expansion compensation may be necessary to ensure acceptable performance over the wide range of temperatures encountered in automotive applications. For example, in the piezoelectric injector, the piezostack has a thermal expansion coefficient of nearly zero, while the steel used in injectors typically has a positive coefficient of thermal expansion. Without thermal expansion compensation, the injector may not operate properly over the required range of temperatures.
It is believed that previous methods of compensating for thermal expansion in fuel injectors may, in certain circumstances, suffer degraded performance and may be inefficient in terms of manufacturing costs. For example, it is believed that previous thermal expansion compensation techniques that rely on hydraulic thermal expansion compensation generally require compensators having closely toleranced internal components and often a check valve assembly, possibly increasing component cost and sensitizing the performance of the compensator to temperature as the viscosity of the hydraulic fluid changes with temperature.
Similarly, use of spring lash compensation techniques to compensate for thermal expansion may require precise heat treatment of the steel and blending of the alloys in order to obtain repeatable performance. Thermal compensation techniques that rely on matching of thermal expansion coefficients of injector components may require precise tolerancing of component lengths to maintain tolerance stackup effects within acceptable limits over a wide range of temperatures.
Thermal compensation techniques using a tail mass with a hydraulic damper rely on inertial damping effects provided by a relatively large tail mass and often require a piston ring or O-ring seal for the hydraulic damper portion. Magnetic clamp thermal compensation techniques are similar to tail mass compensation techniques except that the magnetic clamp compensation techniques substitutes static friction and magnetic clamping force for the inertial damping effect provided by the tail mass, thereby eliminating the need for an O-ring seal around the piston section.
However, it is believed that degraded performance may occur with the tail mass with a hydraulic damper and magnetic clamp approaches, because both of these approaches to thermal expansion compensation typically utilize the fuel available in the injector as the hydraulic fluid. Use of fuel as the hydraulic fluid may reduce damper performance when, for example, the fuel pressure drops to the point that the dynamics of the damper cause cavitation or vaporization of fuel, when the fuel pressure is low enough to cause hot fuel to form vapor bubbles in the damper, in situations where the vehicle is expected to start with very low initial fuel pressure, or when the vehicle is expected to continue to run during fuel system failures that cause the fuel pressure to fall abnormally low. In addition, hydraulic dampers that rely on fuel as the hydraulic fluid may not always open sufficiently to bleed air out of the injector during initial start-up of the vehicle.